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Homework Help: Angular momentum and Springs (2 ques)

  1. Dec 2, 2007 #1
    I have 2 problems that have been giving a bit of trouble since last night when I gave up on them. I found a similar one to the first one on here but I didn't quite understand the equation that was given.

    1. The problem statement, all variables and given/known data
    A student sits on a rotating stool holding two 3.0 kg objects. When his arms are extended horizontally, the objects are 1.0 m from the axis of rotation, and he rotates with an angular speed of 0.75 rad/s. The moment of inertia of the student plus stool is 3.0 kg·m2 and is assumed to be constant. The student then pulls the objects horizontally to 0.30 m from the rotation axis.

    Find the new angular speed of the student in rad/s.


    A 15.0-kg object is attached to a cord that is wrapped around a wheel of radius r = 11.0 cm (Fig. P8.60). The acceleration of the object down the frictionless incline is measured to be 2.00 m/s2. Assume the axle of the wheel to be frictionless. The object is also on a slope of 37 degrees.

    (a) Find the tension of the rope.
    2. Relevant equations
    for the first one I can not figure out what equation to use at all, a boost in the right direction would help me tremendously. I worked on this last night so I don't remember how exactly I got the following answers but here are the wrong answers I came up with: 1.267 rad/s , 2.757 rad/s , 1.688 rad/s .

    for the second, I swear I am going this right but I keep getting the wrong answer.
    For the tension of the rope, at first I tried 15kg(9.81m/s^2)cos(37). But the object is accelerating at 2.00m/s^2 so I tried 15kg(2m/s^2) = 30n. Then I tried both added together. These are the three wrong answers I have gotten 117.5 N, 30 N, and 147.5 N.

    Any help would be greatly appreciated, thank you.

    EDIT: didnt mean to put "springs in the title"
    Last edited: Dec 2, 2007
  2. jcsd
  3. Dec 2, 2007 #2
    I solved the first problem, I was using IiWi = IfWf , sorry for lazy latex. But I didnt consider that the given interia of 3 for the student did not needed to be changed. I was halving it using 1/2MR^2 instead of considering as that whole thing.

    But as for the other problem I am still stumped.
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